1. Field of the Invention
This invention relates in general to magnetic transducers for reading information stored in a magnetic record and, in particular, to a differential magnetoresistive (MR) type magnetic sensor for reading data that has been stored in a magnetic record by vertical recording.
2. Description of the Prior Art
The prior art has disclosed various magnetic sensors for reading data from a magnetic surface by employing the magnetoresistive effect. The operation of the MR type transducer is based on the principle that the resistance of certain materials will change when subjected to a magnetic field. Output signals for such transducers are generated by supplying the magnetoresistive sensing element with a constant electrical current. The flux from the magnetic record that is seen by the sensing MR element will be reflected by a change in voltage that is proportional to the change in resistance of the material caused by the flux.
As discussed briefly in the IBM Technical Disclosure Bulletin, Vol. 15, No. 9, page 2680, MR elements exhibit a linear change in resistance for a given magnetic flux leakage over a relatively small range. The art, therefore, suggests providing a magnetic bias which will center the operation of the range at a point in the linear region. As discussed therein, a pair of separate MR elements are disposed on opposite sides of a centered bias strip which is made of a material such as titanium. Current through the center bias strip, biases each element such that the flux directions are opposite. A bridge current is provided which includes a pair of resistors and the differentially biased MR elements. Signals from the bridge are supplied to a differential amplifier which generates an output signal that is not affected by temperature fluctuations since the common mode rejection principle is operating. The output of the differential amplifier, therefore, reflects the value of the flux being sensed.
U.S. Pat. No. 3,860,965 also describes a magnetic sensor employing a pair of MR strips to obtain common mode rejection of thermal noise. However, in the arrangement disclosed, the separate bias conductor is eliminated and the pair of MR elements operate to mutually bias each other.
U.S. Pat. No. 3,879,760 also discloses a pair of mutually biased MR elements arranged to differentially sense a flux value. In accordance with the disclosed teaching, the MR strips are deposited with their induced easy axes plus and minus 45.degree. respectively relative to their longitudinal axes.
The differential MR devices disclosed in the prior art, including the art discussed above, are basically all four terminal devices in that each end of each strip is connected to a separate terminal. The need for separate connections in thin film technology implementations of MR strips greatly increases the fabrication complexities since several lithographic fabrication steps are required to produce the necessary electrical connections to the two MR elements. Also, since separate fabrication steps are required for each MR element, it is difficult to produce them with identical material characteristics, as required for common mode rejection of thermal noise. Furthermore, alignment problems may occur between the two strips. Finally, it has proven difficult to fabricate the two strips in close proximity while reliably maintaining electrical insulation between them, as is imperative with any such four-terminal device.